Five Tips for Machining Carbon Fiber
The use of carbon fiber reinforced polymers (CFRP) is growing in many industries and as a result there is a higher demand for mechanical testing of composites. To achieve accurate and repeatable results, precisely machined test specimens are required. Due to the nature of the material, carbon fiber machining presents machine shops with new challenges, as compared to machining traditional materials like metals.
Waterjet cutting is a fast and efficient way to cut carbon fiber. One advantage of waterjet cutting is that no special tools are required. It is best to drill a starting hole; otherwise the waterjet can cause delamination. When tight tolerances are required, it may be best to waterjet the part oversized and machine to final dimensions.
Machining of carbon fiber is generally done with a router; however, standard metal machining methods can be used. Machining carbon fiber requires higher spindle speeds than metals, but lower feed rates. The feed rates need to be adjusted to minimize the heat in the part while machining. Carbon fiber has a low thermal conductivity and the majority of the heat remains in the part since there are no chips to help dissipate the heat when machining. This heat from cutting can cause damage to the resin. Since coolant might not be permitted when machining carbon fiber, the tool path and tool must be used to control the heat in the part while machining. Additionally, fracturing of the fibers creates considerable abrasion on the cutting tool, so special tooling is required when machining carbon fiber (see below).
Carbon fiber is abrasive and will wear down tools quickly. When a tool loses its sharpness, it can catch fibers and pull them from the part instead of cutting them. To avoid this, tool life must be monitored during machining as cut quality will decrease quickly. The best way to do this is to monitor the machining time on the tool and change it before it becomes dull.
There are several types and shapes of cutters for different applications, but they are generally made from two materials: carbide and polycrystalline diamond (PCD). Carbide provides good wear resistance, but is more applicable to smaller jobs. PCD cutters offer better wear resistance and are the cutter of choice for several composites machine shops. However, the benefit of PCD cutters does come at a significantly higher cost compared to carbide.
Diamond coated grinding wheels are also a staple of the well-rounded composite machine shop. They allow precise dimensional control, reasonable tool life-expense ratio, and effective heat management.
Drilling is a difficult machining procedure on carbon fiber. Carbon fiber is constructed of layers of material that could easily splinter or delaminate during machining. Drilling speed must be adjusted based on the size and depth of hole. Special drill bits designed specifically for carbon fiber help eliminate delamination during drilling.
The dust from carbon fiber is conductive, and can migrate into electronics and short them out. Hence you must vacuum the dust frequently when machining to avoid causing damage to the machine. Carbon fiber dust can also irritate the skin (similar to working with fiberglass). It is good practice to wear gloves and a dust mask when machining CFRP.
Although it sounds complex, machining carbon fiber is not as difficult as it may seem from the first impression. Once you understand the basics, carbon fiber can be machined to the tight tolerances required for material testing without problems.